1. Field of the Invention
This invention relates to a fluorescent lamp having improved life and efficiency, and more particularly to a fluorescent lamp having electrodes coated with a low volatility emission mix slurry.
2. Description of Related Art
Coating the electrodes in a fluorescent lamp with an emission mix material is well known. Emission mix is typically applied to the electrodes as a slurry comprising an inorganic mixture of barium, strontium and calcium carbonates which is subsequently decomposed during an activation step to the active emission mix oxide material. The most common reason for failure of a fluorescent lamp is the breakdown or depletion of the emission mix oxide on the lamp electrodes.
Current emission mix slurries are suspensions containing about 60 wt. % suspended carbonate solids in a low molecular weight acetate solvent suspension medium including nitrocellulose as a thickener or binder. This slurry is similar to lacquers used in painting applications for centuries, and commonly used in paint lacquer since World War I.
There are several important disadvantages of this type of suspension for emission mix applications. First, commonly used acetate solvents such as isopropyl acetate, butyl acetate, etc., are quite volatile, having high evaporation rates. While desirable for fast drying of lacquers, high volatility presents a problem when coating electrodes. One problem is that the specific gravity of acetate suspensions continually increases during production coating runs due to acetate evaporation. The specific gravity must be readjusted, typically every couple of hours, by adding more binder or solvent to prevent the emission mix coating weight from going out-of-spec. Because the emission mix coating weight is proportional to lamp life, the result is production of fluorescent lamps whose service lives vary as much as 10% from the same batch of emission mix slurry. Consistent lamp life is not possible without continuous readjustment of the specific gravity of the emission mix slurry. In practice even periodic readjustment is not always practiced at lamp plants, so variability in emission mix coating weight and lamp life can be even greater.
Second, acetate-based slurries have low viscosity causing suspended solids to settle very quickly. The slurry must be continuously stirred during production to keep the solids in suspension, which further accelerates acetate evaporation. Otherwise, even assuming uniform overall coating weight, subsequently coated electrodes will exhibit nonuniform deposition of emission mix oxides. This leads to nonuniform electrode activation and unpredictable performance.
A further disadvantage of low viscosity is that the coating quickly wicks to the ends of the electrode upon coating, often plugging the ends while leaving the center of the cathode practically uncoated. This highly nonuniform coating prevents the cathode from being uniformly activated; i.e. resulting in overactivation in the center region of the cathode and underactivation in the heavily coated ends of the cathode.
In addition, the high volatility, combustibility and objectionable odor of acetate/nitrocellulose suspensions present additional safety and OSHA concerns.
There is a need in the art for an emission mix slurry having a solvent/binder system that substantially overcomes the above shortcomings, providing a uniform coating of emission mix slurry throughout a production run. Preferably, such a solvent/binder system will not present significant fire or safety hazards.
A mercury vapor discharge lamp is provided having a light-transmissive glass envelope, an electrode disposed within the glass envelope to provide a discharge, a phosphor layer coated adjacent the inner surface of the envelope, a fill gas of mercury and an inert gas sealed inside the envelope, and a rare earth oxide layer coated over the surface of the electrode. The oxide layer is formed from an emission mix slurry comprising 20-50 wt. % suspension medium and 50-80 wt. % carbonate powder as suspended solids. The suspension medium is selected from the group consisting of (a) organic materials having a vapor pressure of less than 0.1 mm Hg at 20xc2x0 C., and (b) water.
An emission mix slurry is also provided for coating onto a fluorescent lamp electrode. The slurry comprises 20-50 wt. % suspension medium and 50-80 wt. % carbonate powder as suspended solids. The suspension medium is selected from the group consisting of (a) organic materials having a vapor pressure of less than 0.1 mm Hg at 20xc2x0 C., and (b) water.
A method of making an emission mix slurry for coating onto a fluorescent lamp is also provided. The method has the steps of: a) adding zirconia powder to a suspension medium at a rate of less than 1 gram per liter of suspension medium per minute under conditions of vigorous mixing to form a mixture, wherein the suspension medium is polyethylene glycol 200, polyethylene glycol 300, glycerin, ethylene glycol monomer, or a mixture of any thereof; b) vigorously mixing the mixture to uniformly disperse the zirconia powder throughout the suspension medium; c) adding the mixture to carbonate powder under conditions of vigorous mixing to form a slurry, wherein the carbonate powder comprises calcium carbonate, strontium carbonate and barium carbonate; d) vigorously mixing the slurry until the carbonate powder is substantially completely wetted; and e) rolling the slurry on rollers to promote uniform dispersion of the carbonate powder in the slurry, wherein the slurry comprises 20-50 wt. % suspension medium, 50-80 wt. % carbonate powder, and 0.001-10 wt. % zirconia powder.